I have a question regarding CAPWAP Tunneling and the MTU. Couldn't find anything on CCO regarding this topic.
So what's it all about?
A wireless client sends a packet with 1500 Bytes (IP Payload). The Lightweight AP receives that packet an put it inside the CAPWAP Tunnel towards the wireless controller. So, this packet runs over ethernet towards controller.
- IP Header (assuming 20 bytes)
- UDP Header (8 Bytes)
- CAPWAP Header (8 - 16 Bytes)
The CAPWAP payload contains the original wireless frame
- 802.11 Header (24 Bytes)
- Original IP packet (1500 Bytes)
So in total, a packet with 1568 Bytes is sent from the AP to the WLC. In most environments, the maximum MTU is set to 1500 Bytes, so fragmentation occurs (fragmentation is bad ). The packet is fragmented by the AP und assembled back by the WLC.
My point is clear - I want to avoid fragmentation.
Path MTU Discovery:
If I understood the CAPWAP standard (RFC5415) paper correctly (http://www.ietf.org/rfc/rfc5415.txt), then PMTUD is only possible for control traffic (see section 3.5 in the RFC). How should the AP inform a wireless client to decrease the MTU? An ICMP Message Too Big from the APs Management Interface? I don't think so! So I guess PMTUD is not a solution for my problem, right?
Jumbo Frame Support in Infrastructure
My idea is, to enable jumbo frames at the components between the AP and the WLC. My basic question is: Does the AP and the WLC support jumbo frames? If I enable jumbo frames at all my switches and routers, but the AP and the WLC doesn't care about that, it won't solve my problem.
How did you guys solve this? Or is the standard design to ignore the tunnel and let the AP and WLC fragment packets? This would be a little bit dissatisfying.
Thanks in advance
No jumbo frame support on the AP:
<1500-4470> MTU size in bytes
% Interface GigabitEthernet0 does not support adjustable maximum datagram size
Nor is there any jumbo frame support on the WLC side (see CSCsh25990).